1. Field of the Invention
The present invention relates to a fuel cell system comprising a water-permeable-type humidifier. For example, this invention relates to the fuel cell system comprising a humidifier which collects water contained in exhaust gas from a fuel cell, which generates power by the chemical reaction of hydrogen and oxygen, and humidifies the gas supplied to a fuel cell. More particularly, this invention relates to technology which is effective in humidifying a fuel cell at start-up and during the operation thereof.
2. Description of the Related Art
For example, in a fuel cell which uses a solid high polymer membrane as an electrolyte membrane, the solid high polymer membrane must be kept saturated by water in order to obtain the functions of a proton-(hydrogen ion-) conductive electrolyte. In view of this, a water-permeable-type humidifier is provided for collecting water in exhaust gas (off gas) from the fuel cell, and using the collected water to humidify supply gas (hydrogen gas and oxygen gas) which is supplied to the fuel cell.
One type of this water-permeable-type humidifier is a hollow fiber membrane water collecting apparatus comprising a hollow fiber membrane which allows water to permeate parallel to the thickness of the membrane, as for example disclosed in Japanese Unexamined Patent Applications, First Publications Nos. Hei 7-71795 and Hei 8-273687.
In the hollow fiber membrane water collecting apparatus, water is collected from one (high humidity side) of the fluids flowing in and out of the hollow fiber membrane, and the other fluid (low humidity side) is humidified by transferring the collected water through the hollow fiber membrane to the other fluid.
FIG. 28 shows a general fuel cell. As shown in FIG. 28, a fuel cell 201 comprises a cathode (air electrode) and an anode (fuel electrode) on either side of an electrolyte membrane comprising a solid high polymer membrane 201c. A cathode electrode 201 and an anode electrode 201d comprise catalysts, and are provided respectively to the two sides. The solid high polymer membrane 201c for example comprises a proton exchange membrane of perfluorocarbonsulfonic acid membrane, and functions as a proton-conductive electrolyte when saturated by water. At room temperature there is a low specific resistance of 20 xcexa9-proton or less. The catalyst in the cathode electrode 201b creates oxygen ions from oxygen, and the catalyst in the anode electrode 201d creates protons from hydrogen. A cathode side gas passage 201a is provided in the outer side of the cathode electrode 201b, and allows an air supply As of oxidizing gas to flow through. An anode side gas passage 201e is provided in the outer side of the anode electrode 201d, and allows a hydrogen supply Hs of fuel gas to flow through. When the air supply As flows along the cathode side gas passage 201a and the hydrogen supply Hs flows along the anode side gas passage 201e, the effect of the catalyst of the anode electrode 201d ionizes the hydrogen, creating protons. The protons move through an electrolyte membrane comprising the solid high polymer membrane 201c, and reach the cathode electrode 201b. The protons react with the oxygen ions, created from the oxygen of the air supply As by the effect of the catalyst, and thereby create water. The air supply As containing the created water and unused oxygen is exhausted as exhaust air Ae from the cathode side gas passage 201a of the fuel cell 201. On the anode electrode 201d, electrons are created when ionizing the hydrogen. These electrons pass through an external load, such as a motor M or the like, and reach the cathode electrode 201b. 
In the fuel cell system comprising the hollow fiber membrane collecting apparatus, the water which is created when the fuel cell is operated, and exhausted therefrom, is collected via the hollow fiber membrane and used as water for humidifying the fuel cell. In cases where the fuel cell system is switched off for a long period of time and the like, the hollow fiber membrane becomes dry, making it impossible to humidify the fuel cell and to start generating power when the fuel cell is started up.
Not only immediately after start-up but also during normal operation (power-generation) of the fuel cell, there are cases where the amount of humidification obtained by the water-permeable-type humidifier is insufficient, depending on the operational status of the fuel cell (e.g. the humidifying capability of the water-permeable-type humidifier and the changes in the amount of humidification required by the fuel cell). In the abovementioned fuel cell system, the water-permeable-type humidifier collects water from the exhaust air and humidifies the air supply by using the collected water. However, in view of the fact that the evaporation rate of water is inversely proportional to pressure at constant temperature, the amount of water required per fixed volume (capacity) changes when the pressure is changed, even when the predetermined dew-point of the air supply to the fuel cell remains the same. As a consequence, the amount of water acquired from the exhaust air of the fuel cell may not be sufficient for humidification.
By way of example, let us compare (a) a high-pressure operating system which comprises a pump upstream in the cathode side gas passage and shifts the air by positive pressure, and (b) a low-pressure operating system which comprises a pump downstream in the cathode side gas passage and shifts the air by negative pressure. The low-pressure operating system of (b) has less pressure loss than the high-pressure operating system of (a), and superior driving force, but has a drawback that it requires a far greater amount of water per constant volume (capacity). For this reason, the humidification amount of the polymer membrane may be insufficient when using only water acquired from exhaust air, particularly in negative pressure operations, and insufficient humidification of the supply gas may lead to poor power generation while the fuel cell is operating. The degree of humidification insufficiency is noticeable greater than in the case of positive pressure operations.
One conceivable solution would be to increase the scale of the water-permeable-type humidifier, but the inevitable increase in pressure loss makes this impractical.
The present invention has been achieved in order to solve the above problems. It is an object of this invention to provide a fuel cell system which can effectively humidify a fuel cell without excess or insufficiency, both at start-up and during operation of the fuel cell. It is another object of this invention to realize the fuel cell system which can effectively humidify a fuel cell without excess or insufficiency, not only when operating at positive pressure but also when operating at negative pressure, where the humidification is especially likely to become insufficient, and can utilize the economical features of negative-pressure operation.
In order to achieve the above objects, the fuel cell system of this invention comprises a fuel cell (1,101) which generates power by using a supply of fuel gas and oxidizing gas; a water-permeable-type humidifier (a hollow fiber membrane water collecting apparatus 2, a water-permeable-type humidifier 123) which collects water from exhaust gas exhausted from the fuel cell, and humidifies the gas supply to the fuel cell; and an auxiliary humidifier which is provided with a vapor/liquid separator (3, a condenser 126) which separates the water from the exhaust gas, a collected water storage tank (4, a reservoir section of a condenser 126) which stores the separated collected water (9), and an injector (17,126b) which injects the collected water, stored in the collected water storage tank, to the gas supply or the exhaust gas.
Furthermore, this invention provides a fuel cell system comprises a fuel cell (1,101) which generates power by using a supply of fuel gas and oxidizing gas; a water-permeable-type humidifier (a hollow fiber membrane water collecting apparatus 2, and a water-permeable-type humidifier 123) which collects water from exhaust gas exhausted from the fuel cell, and humidifies the gas supply to the fuel cell; and an auxiliary humidifier which collects water that could not be collected by the water-permeable-type humidifier, and humidifies the gas supply to the fuel cell. The auxiliary humidifier is provided with a vapor/liquid separator (3, a condenser 126) which separates the water from the exhaust gas, a collected water storage tank (4, a reservoir section of a condenser 126) which stores the separated collected water (9), and an injector (17,126b) which injects the collected water, stored in the collected water storage tank, to the gas supply or the exhaust gas.
According to these constitutions, water in the exhaust gas which is exhausted from the fuel cell is separated by the vapor/liquid separator, and stored in the collected water storage tank. Then, the auxiliary humidifier uses the collected water in the collected water storage tank to humidify the gas supply to the fuel cell. Consequently, in the case where the gas supply to the fuel cell cannot be humidified by the water-permeable-type humidifier at start-up of the fuel cell, and where the amount of humidification becomes insufficient during normal operation of the fuel cell, the gas supply can be humidified by means of the auxiliary humidifier, which is provided separate from the water-permeable-type humidifier.
In this invention, the auxiliary humidifier may supply the collected water to the fuel cell when the amount of humidification obtained by the water-permeable-type humidifier is insufficient for humidifying the fuel cell (S204). According to this constitution, when the required humidification conditions can be satisfied by the water-permeable-type humidifier alone, the auxiliary humidifier need not be used. This makes it possible to reduce wasteful consumption of power.
Furthermore, in this invention, the auxiliary humidifier may supply the collected water for a predetermined period of time at the start-up (S4) of the fuel cell. According to this constitution, after the water-permeable-type humidifier has become capable of humidification, the system switches from humidification by the auxiliary humidifier to humidification by the water-permeable-type humidifier alone. This makes it possible to reduce wasteful consumption of power.
Furthermore, in this invention, the auxiliary humidifier may supply the collected water based on the difference between the amount of humidification required by the fuel cell and the amount of humidification which can be achieved by the water-permeable-type humidifier. According to this constitution, the amount of water needed by the fuel cell in order to generate power with stability can be added to the gas supply. Moreover, power can be generated with stability.
In this invention, the auxiliary humidifier is provided with a water level detector (10) which detects the water level of collected water stored in the collected water storage tank, and exhausts the collected water from the collected water storage tank when the water level exceeds a predetermined water level. According to this constitution, excess water is automatically exhausted, instead of being left in the system.
Furthermore, in this invention, the auxiliary humidifier may further comprise an anti-freezing apparatus (electrical heater, a water temperature gauge, an auxiliary humidification pipe 13, a humidifying water auxiliary tank 18, a humidifying water auxiliary valve 20, a three way valve 21, and a humidification water circulating pipe for preventing freezing 22) which prevents the collected water from freezing. According to this constitution, the gas supply can be humidified without freezing the collected water, even when the outside air temperature is low.
Furthermore, in this invention, the water-permeable-type humidifier may be provided at least on an air electrode side of the fuel cell, and the auxiliary humidifier collects part of the water created on the air electrode side which could not be collected by the water-permeable-type humidifier, and humidifies the gas supply to a fuel electrode of the fuel cell. According to this constitution, even in the case where humidification on the fuel electrode side becomes insufficient while the fuel cell is operating, the insufficient humidification on the fuel electrode side can be supplemented by using the water which was collected on the air electrode side of the fuel cell.
Incidentally, in this invention, the predetermined period of time may be set to any of a predetermined time from the start-up of the fuel cell, the time until the generated voltage of the fuel cell exceeds a predetermined value, the time until the amount of humidification exceeds a predetermined value, and the time until the dew point of the gas supply exceeds a predetermined value. According to this constitution, the required amount of humidification can be obtained.
In this invention, the auxiliary humidifier may supply water upstream from the water-permeable-type humidifier, or to the gas supply side of the fuel cell. According to the constitution wherein the auxiliary humidifier supplies water to the gas supply side of the water-permeable-type humidifier, the water-permeable-type humidifier is able to humidify the gas supply in the same manner as during normal operation. On the other hand, according to the constitution wherein the auxiliary humidifier supplies water to the gas supply side of the fuel cell, it becomes possible to bypass the water-permeable-type humidifier and directly humidify the gas supply by using the auxiliary humidifier.
Furthermore, in this invention, the auxiliary humidifier may stop the auxiliary humidification when the water level detected by the water level detector is below a predetermined water level. According to this constitution, when the water level of the collected water stored in the collected water storage tank has fallen below the predetermined level, i.e. when there is insufficient collected water for the auxiliary humidifier to carry out humidification, the operation of the auxiliary humidifier is stopped, thereby making it possible to eliminate wasteful consumption of power.
Furthermore, in this invention, the anti-freezing apparatus may be set to operate when the temperature of the collected water falls below a predetermined temperature (e.g. below 3xc2x0 C.). According to this constitution, the anti-freezing apparatus does not operate when the water is not in danger of freezing. Therefore, wasteful consumption of power can be eliminated.
Furthermore, in this invention, an exhaust gas transfer apparatus may be provided downstream from the fuel cell. According to this constitution, the side upstream from the exhaust gas transfer apparatus has low pressure, and the side downstream from the exhaust gas transfer apparatus has high pressure. Consequently, the amount of water required for stable power-generation of the fuel cell is added to the gas supply, and power is generated with stability. Furthermore, since the system operates at low pressure with low pressure loss, the energy efficiency and economic efficiency are increased.
Furthermore, in this invention, the pressure control apparatus (125) which controls the pressure of the exhaust gas may be provided downstream from the vapor/liquid separator. While the fuel cell is operating, the exhaust gas transfer apparatus increases the pressure on the downstream side (exhaust side) of the exhaust gas transfer apparatus to more than the pressure on the upstream side (suction side), and the water obtained by the vapor/liquid separator is naturally supplied. However, cases may be envisaged where the pressure on the side (exhaust side) which is downstream from the exhaust gas transfer apparatus is lower than the pressure which the auxiliary humidifier operates at, or lower than a pressure which obtains sufficient rapid responsiveness. According to the constitution described above, the reliability and responsiveness of the auxiliary humidifier can be increased.